* Allow additional oembed providers to be used (currently all are hardcoded)
* Make per-provider configuration possible
* Choose which finders get used for which provider (eg, force use of oembed for instagram but use embedly for everything else)
* Allow specifying additional parameters for certian providers such as scheme=https to YouTube
This is used by some form fields to convert from the internal Python
value to the value used by a widget, much like `Block.get_prep_value`
for blocks or `Field.get_prep_value` for model fields.